This invention relates to biaxially-oriented polytetrafluoroethylene (“PTFE”) sheets and a method for processing sheet continuously or in reasonably long lengths and widths.
A method for processing PTFE sheet continuously or in reasonably long lengths and widths has been pursued for over 40 years. For instance, in 1961, two patents relating to a method for processing sheet continuously or in reasonably long lengths were issued, the Hartford U.S. Pat. No. 3,003,912 and the Thomas U.S. Pat. No. 3,010,950. However, neither process was eventually successful due to various processing problems and property deficiencies. Quality sheet was prepared in a batch process by Roberts U.S. Pat. No. 3,556,161 method, but only commercialized by Garlock, Inc. Palmyra, N.Y. The parts of the patent dealing with sheet manufacture and the parts of the patent commercialized by Garlock were abandoned by Dupont and therefore not protected by patent.
In 2003, Roberts filed a patent, which is now pending, which embodies a novel technology for producing planar biaxial oriented sheet on a continuous basis with lay-flat stress free qualities. This process does not require multiple pass calendering and fibered reinforcements can be added for true reinforcement.
Art process sheet today is fabricated by compression molding a cylinder, which must be batch sintered (fused), cooled and finally shaved (skived) in a lathe to obtain sheet. Although this process produces a sheet, the sheet will not lay flat, is randomly oriented and is not stress free. Additionally, this art process is not cost effective and is time consuming because filled sheet is costly and of limited quality.
The process of the present invention provides simple improvements over an old uniaxially-oriented art form for manufacturing tubing, tape, film or similar shapes taught in Walker U.S. Pat. No. 2,752,637. This invention utilizes art equipment to produce a uniaxially-oriented pressure coalescible extrudate from which biaxially-oriented sheet and shapes may be fabricated with qualities similar to both the Roberts 161 patent and his pending patent. The present invention can be extremely useful for fabricating many items. Only Roberts' pending patent is capable of processing long high aspect ratio fibers with fibrous PTFE resin to provide true reinforcement to fabricated products. The primary advantage over Roberts 161 is the elimination of multiple calendering steps, which entails more handling time and equipment.
Important changes in the traditional resin preparation procedure of the art process contained in the present invention significantly improves extrusion performance and surprisingly makes it possible to successfully extrude filled compositions, heretofore through to be impossible. Art process paste extrusion limits the amount of filling to very low percentages, up to 5 percent, which is adequate as a marker but is not physically functional and not generally used above 1 percent without interfering with product quality. Filling by the process of this invention can be achieved beneficially up to 90 percent by volume dependent upon the characteristics of the fillers or additives.
The relevant prior art includes the following patents:
Patent No.(U.S. unlessstated otherwise)InventorIssue/Publication Date3,003,912HarfordOct. 10, 19613,010,950ThomasNov. 28, 19613,556,161RobertsJan. 19, 1971